Injections of totally nontoxic calf thymus extracts have been effective in bringing about permanent remission in 10 to 60 percent of inbred Fisher CD rats bearing the otherwise acute lethal monocytic Dunning leukemia. Various chemotherapeutic agents such as chlorambucil, 6MP, etc. can also bring these rats into remission. The kinetics of cell mediated and/or humoral immunity in rats undergoing remission of their leukemias by either or both modalities have not yet been investigated. We propose to investigate the immune response to the Dunning leukemia. Additionally, we would compare the response in animals who proceed to die in comparison with those in various stages of remission initiated by either chemotherapy or thymic extract immunotherapy (both singly and in tandem). Early studies indicate the presence of blocking factors and antibodies to Dunning leukemia cells in the ascites fluids. We hope to characterize and purify these substances. Study of the immune response to this inbred tumor might appear to present an appropriate model for immunotherapy of acute monocytic leukemia in humans.